1. Field of the Invention
The present invention relates to a laser beam printing machine and, more specifically, relates to a laser beam scanning and plotting type printing machine such as laser beam printers (LBP), plain paper copiers (PPC) and laser facsimile machines, in which a deviation in timing between a beam scanning system and a printing data outputting system is reduced.
2. Description of Related Art
A block diagram of a major portion of a conventional laser beam printer is shown in FIG. 6. In the laser beam printer, a laser beam outputted from a laser beam emitting unit 1 is received on a rotatable polygon mirror 4 from which the laser beam is reflected toward a rotatable photo sensitive drum 5. Thereby, the surface of a photo sensitive medium on the photo sensitive drum 5 is scanned by the laser beam. The laser beam intensity at respective moments is varied through control of the transmittancy or ON/OFF of a liquid crystal shutter 3 in response to image information from an image processing circuit 2. As a result, the charged condition at the surface of the photo sensitive medium on the photo sensitive drum 5 is varied, and toners are adhered thereon according to the varied condition in which they are transferred on a printing paper in a density difference to complete a print image.
M represents a motor in which an FG sensor 8 (a sensor which generates a frequency signal corresponding to motor rotation) is built in, and the signal from the FG sensor 8 is to be received by a mirror rotation control circuit 7 to control the rotation of the motor M. Further, numeral 9 denotes a drum rotation control circuit which controls the rotation of the photo sensitive drum 5.
Further, for the sake of convenience of explanation FIG. 6 is illustrated in a block diagram form wherein the circuits are divided depending on their respective functions, however in an actual control unit, the image processing circuit 2, the drum rotation control circuit 9, and the mirror rotation control circuit 7 are already integrated into one microcomputer and are realized by many kinds of program controls. In the drawing, a focusing lens system which focuses the laser beam on the photo sensitive drum 5 is omitted.
The polygon mirror motor is a special motor in which a sensor used for controlling the rotating speed and phase is provided and a mirror is secured thereto, and therefore is a comparatively expensive part. In particular, in this type of motor it is necessary that the respective reflection faces of the polygon mirror (in the present invention the length of the respective reflection faces is important, therefore hereinbelow the reflection faces are referred to as sides depending on necessity), namely the length of the respective sides of the polygon mirror, are equal such that a high processing accuracy thereof is required. This is because, in order to bring about the rotation of the polygon mirror motor into a constant target value, a difference between the target value and values obtained by actual rotation of the respective sides is calculated as an error and the polygon mirror motor is controlled depending on the calculated error.
In this connection, the present assignee has already filed U.S. patent application Ser. No. 08/337,362, now a U.S. Pat. No. 5,754,215, relating to a printing machine using an inexpensive polygon mirror motor in which respective target values are set depending on the lengths of respective sides of the mirror, errors with respect to respective target values are calculated and the rotation of the polygon mirror motor is controlled according to the calculated errors. The corresponding Japanese Patent Application to the above U.S. Patent Application has been already laid-opened on Jul. 21, 1995 as JP-A-7-181409.
Now, when the rotating condition of the polygon mirror 4 reaches to a steady condition, the laser beam printing machine moves into a printing enable condition at this moment, and print data from a host computer (not shown) are transmitted to the image processing circuit 2 provided as an output circuit of printing data. The image processing circuit 2 adds, for example, already stored printing frame data to the transmitted print data and produces data of image information composed of dot patterns of an amount corresponding to one page. Then, these data are stored in a frame memory 2a provided therein and the data corresponding to the first line among the stored data are parallel-loaded into a shift register 2b, where an origin pulse O from an origin sensor is awaited. When a detected origin pulse O is received from the origin sensor 6, a voltage controlled oscillator (VCO) 2d in a PLL circuit 2c is activated. Output pulses from the VCO 2d are applied to the shift register 2b as clock pulses. The shift register 2b serially outputs printing data in response to the received clock pulses. Namely, the image processing circuit 2 sends out, in response to the origin pulse O, image information which is to be outputted in optical form onto the photo sensitive medium on the photo sensitive drum 5.
In response to the clock pulses from the VCO 2d which is controlled by the PLL control loop in the manner explained above, a predetermined number of bit data are outputted from the shift register 2b at a predetermined timing. In response to the output the liquid crystal shutter 3 is driven and the intensity of the laser beam is controlled.
Herein, the oscillation frequency of the PLL circuit 2c is determined in response to the number of print data in one line and is independent from the rotating condition of the motor system. For this reason, with respect to data remote from one near the origin, in particular, the data sent out last, a timing error of about 0.5 clock pulse is caused. Further, when the accuracy of rotation control for the motor is reduced or the rotation thereof is fluctuated, a shear in dot printing position is likely generated. Such possible shear in printing poses a problem, in particular, when the dot density per inch is enhanced for the printing.